1. Field of the Invention
The present invention relates to a method and an apparatus for self-calibration in a mobile transceiver, and more particularly to a method and an apparatus for self-calibrating direct current (DC) offset and imbalance between orthogonal signals, which may occur in a mobile transceiver.
2. Description of the Related Art
In general, basic causes of degrading performance of mobile transceivers include undesired or non-ideal characteristics, such as DC offset and I/Q imbalance.
The DC offset is the effect of self mixing by a mixer in a DC offset wireless receiver. The DC offset occurs when a signal of a Local Oscillator (LO) returns after leaking toward an antenna or when a Radio Frequency (RF) modulation signal input through the antenna is leaked to the LO. The DC offset value generated in this way may saturate a BaseBand (BB) circuit.
The I/Q imbalance is caused when the phase difference between the in-phase channel signal (I channel signal) and the quadrature-phase channel signal (Q channel signal) generated in an oscillator of a wireless transmitter is not 90 degrees. The I/Q imbalance can be reduced by designing mixers of the I channel demodulator and the Q channel demodulator to be symmetric to each other. However, designing the mixers to be symmetric to each other requires increases in the volume and current consumption of the mixers. Such I/Q imbalance increases the Bit Error Rate (BER), thereby degrading the performance of the wireless transceiver.
Therefore, in order to improve the performance of a wireless transceiver, it is necessary to arrange a solution for compensating for the DC offset and the I/Q imbalance.
FIG. 1 illustrates a representative example of a process for self-estimating and self-calibrating a DC offset and an I/Q imbalance which occur in a conventional wireless transceiver. The example shown in FIG. 1 is disclosed in a Patent Cooperation Treaty (PCT) application No. 2004/023667, entitled “Direct conversion transceiver enabling digital calibration,” and a paper by James K. Cavers, entitled “New Methods for Adaptation of Quadrature Modulators and Demodulators in Amplifier Linearization Circuits.” For the convenience of description, the estimation path is not distinguished into an I channel path and a Q channel path. However, the same application is possible even when the estimation path is distinguished into an I channel path and a Q channel path.
The solution proposed in FIG. 1 calibrates both the I/Q imbalance and the DC offset generated at a transmission (TX) side and a reception (RX) side. To this end, the calibration for the TX side is first performed, and the calibration for the RX side is then performed. The calibration for the TX side corresponds to imbalance calibration between I channel and Q channel (TX IQ calibration). The calibration for the RX side includes calibration for the DC offset as well as the imbalance calibration between the I channel and the Q channel.
Further, for the calibration for the RX side, the calibration for the TX side must first be performed. In other words, the calibration for the RX side is possible only after the calibration for the TX.
For the calibration for the TX side, an estimation of the DC offset and the I/Q imbalance at the TX side must first be performed. In the estimation shown in FIG. 1, a discrete detector is used. The discrete detector converts an envelope signal output from a drive amplifier of the TX side into a BaseBand (BB) signal and performs a discrete Fourier series for a complex envelope waveform of the BB signal. Based on the discrete Fourier series, the discrete detector estimates the gain imbalance, the phase imbalance, and the DC offset of each of the I channel and the Q channel at the TX side.
However, in the case of the estimation as described above, it is necessary to incorporate the non-ideal factors, which include a differential gain and a DC value. In the above-mentioned paper and patent application, the non-ideal factors are estimated.
Therefore, the TX and RX gain imbalance, phase imbalance, and DC offsets of the I channel and Q channel obtained through this estimation may be incorrect. Further, as noted from FIG. 1, many separate diodes, registers, capacitors, and switches are necessary in order to construct the discrete detector.